1. Field of the Invention
The invention relates to a lens module and in particular to a fixed-focus lens module.
2. Description of the Prior Art
US patent pub. No. 2007/0223102, U.S. Pat. No. 6,075,658, and TW patent Nos. I315410, M269477, I307813, and M362997 disclose many kinds of lens modules. However, a lens module used in a projector is required to meet the requirement of high image quality so an image projected by the lens module is generally required to meet the requirements of low distortion aberration, high resolution, high contract, high uniformity and so on. Further, in order to project a large image in a short distance, the lens module is required to meet the requirement of larger field of view (FOV) and the image projected by the lens modules is required to meet the requirement of small keystone aberration. In addition, in order to increase utilization of light and uniformity of brightness of the projected image, a maximum angle between a main light at a reduced side of the lens module and an optical axis, that is telecentric angle, could be as small as possible such that the main light is almost parallel to the optical axis.
In order to design the above lens module which meets the requirements, many difficulties need to be overcome. For example, in order to reduce the distortion aberration, the FOV is reduced and the number of the lenses of the lens module is increased. In order to increase the FOV and reduce the telecentric angle, the length of the lens module is likely increased and the size of each of the lenses is likely enlarged. In the conventional art, in order to design the lens module having the effective focal length with about 10 millimeters and the image quality meeting above requirements, the number of the lenses is excess such that the space occupied by the lens module is enhanced and the production cost of the lens module is increased.